1. FIELD OF THE INVENTION
The present invention relates to a process for modulating the effect of the speed of moving parts of a body in a density measurement by nuclear magnetic resonance (NMR), as well as the performance of the process for deducing therefrom the speed of the moving parts in question. The invention is more particularly used in the medical field, where the bodies examined are human bodies and where the moving parts are cells of the blood circulating in the veins and arteries, or moving organs such as the cardiac muscle. In this application, the invention can be more particularly realised with an imaging or image production process in order to give an image representing the distribution of the speeds of the moving parts in a section of the body examined.
Thus, imaging by nuclear magnetic resonance is mainly developed as a medical diagnosis means. It makes it possible to visually display internal tissue structures with a contrast and a resolution level never simultaneously achieved before with other image production processes.
In order to obtain an image by nuclear magnetic resonance of a section of a body with differentiation of the tissue characteristics thereof, use is made of the property of certain particles, such as protons, of orienting their magnetic moment whilst acquiring energy when they are placed in a constant main magnetic field B.sub.O. A particular zone of a body containing particles then has an overall magnetic moment which can be tilted or flipped in accordance with a given orientation, perpendicular or parallel to field B.sub.O, by inducing a resonance by the emission of a radiofrequency magnetic field perpendicular to the main field.
All the particles which then have a magnetic moment rotating at a so-called Larmor precession speed, tend to find again the initial orientation parallel to B.sub.O by emitting a radiofrequency signal at the characteristic resonant frequency of B.sub.O and of the particle. This signal can be detected by a receiving antenna. The duration of the return to equilibrium of the overall magnetic moment of a region in question and the decrease of the signal are dependent on two important factors, the spin-system interaction and the spin-spin interaction of the particles with the surrounding material. These two factors lead to the definition of two relaxation times characteristic of the tissue and respectively called T.sub.1 and T.sub.2. A considered region of an object consequently emits a signal, whose intensity is dependent on T.sub.1, T.sub.2, the density of the particles in the region and the time which has elapsed since radiofrequency excitation.
2. DESCRIPTION OF THE PRIOR ART
If the orienting field is perfectly homogeneous, in response, mobile particles in a considered region emit a signal identical to that of the fixed particles of said region. However, if the orienting field is not homogenous, or, more generally if for various reasons (particularly for carrying out image formation) during or after radiofrequency magnetic excitation, an interfering magnetic field is applied which has an intensity gradient, it is possible to show that the contributions made by the mobile particles in the overall signal emitted are affected by a phase component dependent on the speed thereof. This can be easily understood. The resonant signal emitted vibrates at a frequency f.sub.O, which is dependent on the intensity of the orienting magnetic field B.sub.O and a gyromagnetic ratio characteristic of the medium in question.gamma.. All variations in the intensity of the field B.sub.O consequently lead to a corresponding variation of the resonant frequency. Consequently a fixed particle which, following radiofrequency excitation, is exposed firstly to the field B.sub.O resonates at a frequency f.sub.O and then secondly is exposed to a stronger field B.sub.O +.DELTA.B.sub.O, it resonates at a higher frequency f.sub.O +.DELTA.f.sub.O. Thirdly it is again exposed to field B.sub.O and it again vibrates at frequency f.sub.O. During the latter the signal emitted is then phase displaced with respect to its phase initially. This phase displacement is proportional to the amplitude of the interference .DELTA.B.sub.O and to the duration of said interference. If all the particles of the medium does not have a gradient, this simply means that the overall signal emitted is delayed.
However, the procedure is quite different in the case of particles having a certain speed when the interference has a gradient. During three periods and as a result of the displacement speed thereof during these periods, they occupy regions in space where the orienting and interfering fields differ. They differ respectively as a result of the existence of inhomogeneities or the fact that gradients exist. Therefore the contribution of the mobile particles in the signal is provided with a phase dependent not only on the amplitude of the interference encountered (as for fixed particles), but also the amplitude variation of said interferences along the path which they have taken. This variation, which constitutes the gradient is geographically imposed. Consequently the phase displacement of the signal of the mobile particles is then dependent on their speed, because the higher their speed the more regions in space they occupy. If the displacement speeds, inhomogeneity or field gradients are too large, the phases of the different contributions can be affected at this point and end up by providing opposition. In this case, these contributions are mutually cancelled out and the resulting overall signal is not as strong. In practice this effect is such that it often gives the illusion that there is no matter in a body at the location where the mobile particles circulate.
To reveal the existence of mobile particles and to measure their characteristics, the density and possibly the displacement speed, it is possible to proceed in accordance with a method described by E. L. HAHN in February 1960 in the Journal of GEOPHYSICAL RESEARCH, vol. 65, no. 2, p. 776 ff. The author suggests subjecting the medium in question to a sequence of a particular gradient and coding it. The principle of this coding consists of applying following the flipping of the radio-frequency pulse, a bipolar gradient along the axis of a velocity component which it is wished to recognise. A bipolar gradient is such that its time integral is zero from the time corresponding to the start of the radiofrequency pulse to the time corresponding to the measurement. The magnetic moment of the spin of a stationary particle in this case only undergoes a zero overall phase displacement. Thus, the phase displacement undergone during the application of the first part of the bipolar gradient is compensated by the application of the second part of said gradient. However, a mobile particle with a positive speed along the gradient axis then undergoes during the second part of the pulse, a larger phase displacement in absolute values than during the first part. The reason is that during this second part, it frequents a region in space where, due to the gradient, the interfering magnetic field is stronger. By comparing a measurement made with such a bipolar gradient and a measurement made without it being applied, it is possible to deduce therefrom the speed and number of mobile particles.
Whatever the objectives pursued, simple measurement or measurement with an image and no matter what the procedures adopted, the sensitivity of the speed phenomenon to the interfering magnetic field applied is such that the displacement phenomena can only be revealed when the maximum speeds are below a limit. Particularly in image formation, depending on whether the velocity component to be revealed is parallel or perpendicular to the plane of the imaged section, the sensitivity of NMR machines is at present approximately 1 radian (cm/s) to 0.2 radian (cm/s). This means that a particle moving at 1 cm/second in the plane of the section contributes to the overall signal emitted with a phase displacement of 1 radian compared with the contributions emitted by the fixed particles. In the human body a nominal blood circulation speed of 50 cm/s is reached at present, whereby it can even be several metres per second in the heart. Moreover, the distribution of the speeds in a vessel ranges between zero on the edges of the vessel and nominal speed at the centre of the vessel. Thus, each particle of a vessel contributes to the signal with a phase displacement which can be zero to 50 radians. Knowing that contributions phase displaced by .pi. radians mutually oppose one another, the resulting signal is zero, which amounts to taking the mean value of a sinusoidal signal over several periods or cycles. For example, Paul R. Moran in an article in Radiology of RSNA, 1985, 154, pp. 433-441 refers to a measurement of a mean speed equal to 0.6 cm/s and corresponding to a phase displacement of approximately 90.degree.. Beyond this limit, the sensitivity of the machines is too great and the speeds can no longer be measured.